The solders generally used in industry are alloys of tin, lead and zinc. Tin-lead solders are used to join most of the metals as they have good corrosion resistance to most of the metals. Sometimes silver, cadmium, antimony and bismuth are also added to solders. It is essential that solders must have a lower melting point than the parent metals. The melting point of a solder should be as near as possible to the parent metal to be joined so that a more tenacious joint is affected.
Composition of Soft Solder
Soft solders are mostly alloys of lead and tin. All the plain tin / lead solders become solid at 183C. The temperature at which these become completely liquid depends upon the composition. The temperature increases as the lead content increases or tin content decreases.
Some solders pass through a considerable pasty stage before becoming completely liquid to solid. The above figure shows that 20% tin and 80% lead solder becomes completely liquid at 276C and solid at 183C. The solder passes through a pasty stage in the transformation from liquid to solid. This feature is useful in some plumbing applications or where work is coated by dipping in a bath of molten solder.
A solder which contains 60% tin and 40% lead has a lower melting point. It is in solid state at 183C and becomes completely liquid at 188C. So it is especially used in electrical work where a longer cooling period could damage insulation or the component.
Sometimes a little percentage (maximum up to 3%) of antimony is also added to make harder and stronger joint because it expands on cooling. A solder containing antimony is not recommended for zinc and galvanised work. Bismuth is also added to soft solder to lower down the melting point.
Solder Compositions as per Indian Standard (IS): 193-1982
IS:193-1982 covers the requirements of non antimonial and antimonial soft solders for general use, solders for use at high temperature and solders for special applications.
(a) Tin-Lead Solders for General Use. These can be non-antimonial solders and antimony solders.
(i) Non-antimonial Solders. These solders contain a very small percentage of antimony. Some examples are as follows:
|Tin %||Antimony % (max)||Lead %|
|65 ± 0.5||0.65||Remainder|
|50 ± 0.5||0.50||Remainder|
|20 ± 0.5||0.20||Remainder|
(ii) Antimonial Solders. These solders contain good percentage of antimony. Some examples are as follows:
|Tin %||Antimony % (max)||Lead %
|50 ± 0.5||2.5 – 3.0||Remainder|
|32 ± 0.5||1.6 – 1.9||Remainder|
|18 ± 0.5||0.9 – 1.2||Remainder|
|02 ± 0.5||0.2 – 0.5||Remainder|
(b) Solders for Services at High Temperature. These solders are more difficult to use. Some examples are as follows:
Solders for Services at High Temperature
|95 ± 0.5||4.75 – 5.25||0.07 Max||0.02 Max|
|4.75 – 5.25||0.10 Max||Remainder||1.4 – 1.6|
|0.75 – 1.25||0.10 Max||Remainder||1.4 – 1.6|
Forms of Soft Solder. Soft solders are supplied or manufactured in various forms such as bars, strips, tapes, solid wires, cored wires, ingots, washers, pastes, paints, etc.
Special Type of Solders
Resin Core Solder. Normally the solder and flux are two separate things but in resin core, solder is prepared in such a way that it is made in the form of thin tube and flux is filled inside the tube (i.e. core). In this, the solder and flux both are available at the same time. Soldering with resin core solder is easy, less time taking, clean and has more strength. It is usually used in soldering of electrical, radio, computer and TV components.
Standard Grades of Soft Solder
Standard grades of solder including those provided in service are given below. All these standard grades start to melt at about 185 C. But special solders whose melting point is up to 310 C are also available. The paint solders are applied with brush. Some standard grades of solder are given in table as follows:
Grades of Soft Solder
|Melting point (C)||Uses|
|B||47.5||50||2.5||…||225||Aircraft general use|
|C||58-59||39-40||2.24||…||227||Pipes, radiators, coolers and general work|
|Plumber||D||69-70||29-30||1.17||…||248||High grade plumbing and lead cables|
|G||58-59||41-42||0.4||…||230||Zinc galvanized iron, electrical and general works|
|Electrical||M||53-54||44-45||2.7||…||215||Electrician, tinsmiths and instrumental works|